This invention relates to current regulators and more particularly to current regulators for supplying relatively low currents at relatively high voltages.
Certain applications require that a relatively low current of, for example, from 20 microamps to 500 microamps, be accurately regulated and supplied to a load at a relatively high voltage of, for example, 3 kilovolts. One such application is in a focus supply for driving cathode ray tubes, for example. High voltage transistors are available for relatively high voltages, such as 1.5 kilovolts. But the leakage currents of these transistors exceed the desired currents to be supplied to these particular loads. The prior art has addressed this problem of controlling relatively low currents at relatively high voltages by stacking transistors to achieve the high voltage control. Conventional transistors, however, are susceptible to a phenomenon called second-breakdown, brought about by exceeding the collector-emitter voltage ratings. To prevent this from happening, the stacked transistors have to be surrounded with protective components. This adds undesirable size and complexity to the prior art current regulators.
It is sometimes desired, instead of supplying a constant current to a load, to modulate that current. Such modulation is required, for example, in dynamic focusing for cathode ray tubes. In such tubes the scanning beam must be accelerated at the edges of the tube, which requires a change in the focusing current. Adding such a feature to conventional current regulators only adds to the already complex circuitry.